The invention relates to a method for measuring the flow rate of a medium in a volume permeated by a magnetic field, wherein an electric field is induced in the medium by its flowing motion. More particularly, the invention relates to signal decoupling in a magnetic-inductive flow rate sensor (MID).
Most of the magnetic-inductive flowmeters or flow rate sensors available on the market today operate with galvanic signal decoupling and require a minimum electrical conductivity of the medium. Measuring the induced voltage here, in practice, amounts to a current measurement, which becomes more difficult the greater the internal resistance of the medium. In addition, this type of coupling requires a time variable magnetic field, in order to eliminate disruptive electro-chemical potentials on the metallic, conductive electrodes.
The systems available on the market with capacitive signal decoupling, which is advantageous for many reasons, also cannot eliminate a minimum conductivity, because in the end a current measurement is also performed in these systems. In contrast to galvanically coupled systems, for basic reasons, a time variable magnetic field is necessary in these systems to generate a continuous, alternating current flow.
An alternative is presented, however, in German Patent DE 102 21 677 C1, in which the time variable magnetic field is replaced by time variable coupling capacitors. This opens up the possibility of using permanent magnets and thus reducing to zero the considerable power requirements for generating a magnetic alternating field. However, this alternative—just like the systems described above—is subject to a minimum conductivity of the medium, because here the current generated by the variable capacitors is also measured.
Another solution approach for using the magnetic-inductive measurement method with non-conductive media is described in German published patent application DE 198 43 808 A1, in that there the induced voltage is not used directly as a measurement parameter, but instead the resulting dielectric polarization charge is used. Indeed, here no minimum conductivity of the medium is required. However, in the end this method also depends on measurement of the displacement current caused by the time variable polarization charge. Due to the small magnitude of this polarization charge, realization of this measurement is difficult.
Furthermore, the prior art includes the arrangement described in German Patent DE 199 22 311 C2 for determining three-dimensional rate distributions in electrically conductive liquids, the device shown in the Japanese laid-open publication JP 01-178822 A for measuring the flow rate with a semiconductor device, the electrode arrangement shown in German published patent application DE 2 401 641 A1 for an electromagnetic flowmeter, and devices described in the article by Tewodros Amare, “Design of an electromagnetic flowmeter for insulating liquids,” Meas. Sci. Technol. 10:755-758 (1999).
Up to the last-cited reference of T. Amare, the corresponding prior art methods, without exception, are only successful with the help of a current measurement, which requires a minimum conductivity of the medium in practical measurement systems available today. Therefore, existing systems are limited to the use of aqueous media.
The last-cited reference of T. Amare assumes that the resistance, or current flow, between each electrode and a screening electrode is measured for determining its capacitance. Here, the magnetic field frequency and the field set electrostatically are difficult to control.